The countdown to spring football practice is on.
Over the next four weeks, in advance of Nebraska's March 8 open to spring practice, ESPN.com will address important offseason topics for the Huskers.
This week, we'll identify the position groups with most room to improve. Up first, the secondary:
Top returnees: Rising seniors Josh Mitchell, a cornerback, safeties Corey Cooper and Harvey Jackson, sophomore safety LeRoy Alexander and junior Charles Jackson, a versatile defender who could fit in several spots.
Numbers to know: Nebraska extended its historically strong performance under coach Bo Pelini in limiting completion percentage of its opponents. Last season, foes connected on 54.1 percent -- a notable slip from the Huskers' nation-leading 47.1 percent in 2012, but good enough to rank 19th nationally and third in the Big Ten. The Huskers allowed 214.6 yards per game through the air, fourth in the league and 33rd nationally. They also ranked ninth in the Big Ten by surrendering 7.2 yards per pass attempt.
Key question: How will the defensive backs adjust to another new position coach, with Charlton Warren as the fourth assistant to lead this group on the past five years?
The outlook: There's plenty of room for growth. And good news for the Huskers, the need for improvement in the secondary is outweighed by the potential to make it happen. Nebraska needs a big final season from Mitchell, who set the stage for it with an interception and a fumble recovery in the Huskers' Gator Bowl win over Georgia. And the Huskers need a safety to step up in the spot alongside Cooper, who led Nebraska in tackles as a junior.
Junior Daniel Davie and redshirt freshmen Boaz Joseph, Drake Martinez and D.J. Singleton will get a shot to emerge, though the top candidate to make a huge leap is junior Jonathan Rose -- who played a small role in 2013 after transferring from Auburn.
The Huskers are deep enough here, barring a string of injuries, to avoid the need for much help from freshmen. But if anyone among the group that includes Luke Gifford, Chris Jones, Trai Mosley and Josh Kalu shows the readiness to break the two-deep, the door is always open.